During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics, sometimes referred to as paper machine clothing (PMC), all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process, which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Flat woven fabrics can also be formed into engineered fabrics. Whether the flat woven fabric is a forming, press, dryer, through-air-drying (TAD) or an engineered-fabric, used, including but not limited to, to produce nonwovens by processes such as spunbonding or hydroentangling, or for wet processes such as a DNT belt or sludge filter belt or the like, or textile finishing belts, the properties of seam uniformity and integrity are critical.
Fabric seam terminations or the ends of the yarns that are interlaced or interwoven to form the seam are susceptible to pulling back when run on a paper, TAD or other industrial machines when the fabric is subjected to machine direction (MD) tension. To minimize this seam “pullback”, the terminal ends of the yarns in the seam are sometimes bonded to an adjacent yarn with an adhesive. However, adhesives are not fully resistant to the machine running conditions, and still allow for pullbacks or yarn slippage to occur over time. Likewise, the use of adhesives with other reinforcement means such as sewing terminal ends of a PMC, TAD or engineered fabric does not produce the desired seam integrity or uniformity.
In addition, the width of the seam area, as measured in MD, formed using conventional techniques typically range, for example, anywhere between three and one half to twenty inches or even more. For many reasons, it is desirable to reduce the seam area.
FIGS. 1(a-d) show the problems associated with conventional seam formation techniques, wherein the terminating ends of the two fabric edges are “overlapped” in the seam area and the critical points 12, where these ends might “pullback” in the MD and the ends themselves might protrude through the paper side surface, are identified (FIG. 1a). Eventually, the slippage in the overlapping area increases as shown by the arrows due to increased localized stresses in the fabric (FIG. 1b) and there is a complete slippage and a hole 16 appears in the seam area of the fabric (FIG. 1c). Accordingly the overlap region of the seam is typically reinforced by manually gluing 18 (FIG. 1d) to increase its strength; however, gluing is a laborious and time consuming process. Due to its low precision, it is also hard to limit the glue to only the overlapping yarns. In addition, the glue eventually either fails due to flexing of the fabric and/or abrasion.
Accordingly, there is a need for a different or improved means of strengthening seam yarn end terminations, and consequent seam strength.
A possible technique for strengthening seam yarn end terminations for fabrics is welding, such as ultrasonic welding. Ultrasonic refers to sounds that are above the range of human hearing, i.e. >20,000 Hz and ultrasonic welding refers to the fusing of materials with sound waves. However, unacceptable seam formation results often arise from ultrasonic welding such as excessive melting of the yarns, reduced seam permeability, and distortions in the fabric due to localized shrinkage.
Other seaming methods can be found in the following references:
U.S. Pat. No. 4,032,382 relates to a method and apparatus for splicing thermoplastic monofilament material. Two filaments are joined together in a channel in an apparatus. They are exposed to high frequency vibrations under lateral pressure in the channel, which dimension corresponds to the original diameter of the filaments. The outcome is a splice with a diameter close to the original material.
U.S. Pat. No. 4,401,137 relates to a forming fabric with a non-marking looped pin seam with trimmed “tails” which may then be ultrasonically welded to increase the strength of the seam.
U.S. Pat. No. 4,501,782 relates to an alternative method to join the fringed fabric warp ends in a completely rigid manner. The warp ends are put in interdigitated position and ultrasonically bonded in a continuous manner across the fabric. The seam joint claims to have the same caliper as the main body of the fabric after this operation.
However, this seam is not acceptable for the demands in papermaking as it causes marks in the paper and/or gives uneven drainage. The risk for paper web breakage is high if such seam is used for a TAD fabric as the tissue web is so thin.
U.S. Pat. No. 5,464,488 relates to a method for forming a strong and flexible bond between two plastic fabric layers that requires the controlled use of an ultrasonic welding apparatus. The yarns are said to soften, but not melt, and bind to each other during this process. The bond becomes flexible and useful as it is not brittle, and consequently not susceptible to the cracking and breakage in the bonds presented in the prior art.
However, this seam is not useful in open structures like TAD fabrics. A seam more like that of the traditional seams with properties close to the main body of the fabric is essential for the performance. The proposed seam is of that improved kind.
U.S. Pat. No. 5,571,590 provides papermaking fabrics with cross machine direction (“CD”) seams formed by superposed end portions of a flat woven substrate element with end portions extending away from the ends in a common direction; concurrently the ends are severed from the end portions, and the end potions are fuse bonded along a common cut line.
U.S. Pat. No. 5,713,399 provides a PMC manufactured by spirally winding a woven fabric strip narrower than the intended width of the fabric. The strip has a lateral fringe of unbound ends of crosswise yarns on at least one lateral edge. This fringe underlies or overlies the edge of the adjacent turn of the strip. The spirally continuous seam so obtained is closed by ultrasonic welding.
U.S. Pat. No. 5,731,063 provides papermaking felts with cross machine direction seams formed by end portions of a flat woven substrate element with end portions extending away from the ends in a common direction. Concurrently the ends are severed from the end portions, and the end portions are fuse bonded along a common cut line, similar to U.S. Pat. No. 5,571,590.
U.S. Pat. No. 6,162,518 relates to a spirally wound PMC similar to U.S. Pat. No. 5,713,399. At least one connecting thread is placed over the projecting thread sections of the edges before the heat bonding is initiated.
U.S. Pat. No. 6,702,927 relates to a PMC manufactured by spirally winding a woven fabric strip narrower than the intended width of the PMC. The fabric strip has first and second lateral edges, along which are a first and second lip, respectively. Each lip has at least one lengthwise yarn woven with crosswise yarns. Adjacent to and inward of the first and second lips are a first and second gap, respectively, which lack lengthwise yarns but whose crosswise yarns join the lips to the body of the strip. When spirally wound the lips overlay the gaps of adjacent turns to form a seam which may be closed by ultrasonic bonding or other means.
Published U.S. Patent Application No. 2003/0221739 relates to a PMC using at least one preformed loop or coil in a pin seam joining the ends of the fabric to form an endless belt. The ends of the base fabric may be pre and/or post treated by compaction, pre-squeezing and sealing to stabilize the ends. Ultrasonic melting or other means may be used also to reinforce and provide a smooth surface in the stitched area.
While ultrasonic welding has several advantages over conventional seam formation techniques such as sewing or gluing, certain drawbacks of previous ultrasonic welding techniques described above stemmed in part from the fact that conventional ultrasonic welding is based on modifying multiple parameters of time, energy and distance. Therefore, there is still a need in the art for a means of creating fabric seams with acceptable surface smoothness and strength which is also easy to use and applicable to a broad range of fabric types.
Note that citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.